arXiv subjects

T. Baeva

Publications and source records attributed to T. Baeva.

6 recordsLinked to original sources

arXiv2010

Zero vector potential mechanism of attosecond absorption in strongly relativistic plasmas

The understanding of the physics of lasermatter interactions in the strongly relativistic regime is of fundamental importance. In this article, a new mechanism of fast electron generation at the vacuum-solid boundary of intense laser pulse interaction with overdense plasma is described. It is one that has no analogue in classical, non-relativistic laser-plasma interactions. Here, conclusive proof is provided that the key contribution to the fast electron generation is given by the zero points of the vector potential. We demonstrate that the new mechanism leads to scalings for the fast electron energy, which explicitly depend on the plasma density, thus providing a new insight into relativistic laser-matter interaction. Furthermore, it is shown that this new mechanism provides the dominant contribution to the interaction by the injection of energy into the overdense plasma delivered by attosecondduration electron bunches. This new understanding will allow the future generation of a single ultra-bright attosecond X-ray pulse by suitable control of the laser pulse polarization [1]-[7]. This process will also allow single pulse attosecond electron bunches to be generated that can be further accelerated in laser wakefield accelerators [8]-[10]. Other applications that will benefit from this new insight include laser driven ion accelerators [11]-[15], fast ignition inertial confinement fusion [16]-[18] as well as fundamental studies at the intensity frontier [19]-[20].

physics.plasm-ph↗

arXiv2006

Theory of high harmonic generation in relativistic laser interaction with overdense plasma

High harmonic generation due to the interaction of a short ultra relativistic laser pulse with overdense plasma is studied analytically and numerically. On the basis of the ultra relativistic similarity theory we show that the high harmonic spectrum is universal, i.e. it does not depend on the interaction details. The spectrum includes the power law part $I_n\propto n^{-8/3}$ for $n<\sqrt{8α}γ_{\max}^3$, followed by exponential decay. Here $γ_{\max}$ is the largest relativistic $γ$-factor of the plasma surface and $α$ is the second derivative of the surface velocity at this moment. The high harmonic cutoff at $\propto γ_{\max}^3$ is parametrically larger than the $4 γ_{\max}^2$ predicted by the ``oscillating mirror'' model based on the Doppler effect. The cornerstone of our theory is the new physical phenomenon: spikes in the relativistic $γ$-factor of the plasma surface. These spikes define the high harmonic spectrum and lead to attosecond pulses in the reflected radiation.

physics.plasm-ph↗

arXiv2006

Relativistic plasma control for single attosecond pulse generation

To describe the high harmonic generation at plasma surfaces in the relativistic regime, we introduce the concept of {\it apparent reflection point} (ARP). It appears to an external observer that the radiation electric field is zero at the ARP. The relativistic dynamics of the ARP completely defines the generation of high harmonics and attosecond pulses. The ARP velocity is a smooth function of time. The corresponding $γ$-factor, however, has sharp spikes at the times when the tangential vector potential vanishes and the surface velocity becomes close to the speed of light. We show that managing the laser polarization, one can efficiently control the ARP dynamics, e.g., to gate a single (sub-)attosecond pulse out of the short pulse train generated by a multi-cycle driver. This relativistic control is demonstrated numerically by particle-in-cell simulations.

physics.plasm-ph↗

arXiv2005

Focusing of laser-generated ion beams by a plasma cylinder: similarity theory and the thick lens formula

It is shown that plasma-based optics can be used to guide and focus highly divergent laser-generated ion beams. A hollow cylinder is considered, which initially contains a hot electron population. Plasma streaming toward the cylinder axis maintains a focusing electrostatic field due to the positive radial pressure gradient. The cylinder works as thick lens, whose parameters are obtained from similarity theory for freely expanding plasma in cylindrical geometry. Because the lens parameters are energy dependent, the lens focuses a selected energy range of ions and works as a monochromator. Because the focusing is due to the quasineutral part of the expanding plasma, the lens parameters depend on the hot electron temperature $T_e$ only, and not their density.

physics.plasm-ph↗

arXiv2004

Coherent Harmonic Focusing and the Light Extreme

We demonstrate analytically and numerically that focusing of high harmonics produced by the reflection of a few femtosecond laser pulse from a concave plasma surface opens a new way towards unprecedentally high intensities. The key features allowing for boosting of the focal intensity is the harmonic coherency and the small exponent of the power-law decay of the harmonic spectrum. Using the similarity theory and direct particle-in-cell simulations we find that the intensity at the Coherent Harmonic Focus (CHF) scales as $I_{\tiny CHF} \propto a_0^3 I_0$, where $a_0$ and $I_0\propto a_0^2$ are the dimensionless relativistic amplitude and the intensity of the incident laser pulse. The scaling suggests that due to the CHF, the Schwinger intensity limit can be achieved using lasers with $I_0 \approx 10^{22}$ W/cm$^2$. The pulse duration at the focus scales as $τ_{\tiny CHF} \propto 1/a_0^2$ and reaches the subattosecond range.

physics.optics↗

arXiv2004

Relativistic Doppler effect: universal spectra and zeptosecond pulses

We report on a numerical observation of the train of zeptosecond pulses produced by reflection of a relativistically intense femtosecond laser pulse from the oscillating boundary of an overdense plasma because of the Doppler effect. These pulses promise to become a unique experimental and technological tool since their length is of the order of the Bohr radius and the intensity is extremely high $\propto 10^{19}$ W/cm$^2$. We present the physical mechanism, analytical theory, and direct particle-in-cell simulations. We show that the harmonic spectrum is universal: the intensity of $n$th harmonic scales as $1/n^{p}$ for $n < 4γ^2$, where $γ$ is the largest $γ$--factor of the electron fluid boundary, $p=3$ and $p=5/2$ for the broadband and quasimonochromatic laser pulses respectively.

physics.plasm-ph↗